1. Field of the Invention
This invention relates to a method for producing a bonded wafer, and more particularly it is to effectively prevent the occurrence of flaws on front and back faces of the wafer to improve a thickness uniformity of a top layer at an outer peripheral portion of the wafer.
2. Description of the Related Art
As a typical production method of the bonded wafer, there are known a method wherein a silicon wafer having an oxide film (insulating film) is bonded to another silicon wafer and then one side of the bonded wafer is ground and polished to form SOI layer (grinding-polishing method), a method wherein oxygen ions are implanted into an interior of a silicon wafer and then a high-temperature annealing is conducted to form a buried oxide film inside the silicon wafer and then an upper part of the oxide film is rendered into SOI layer (SIMOX), a method wherein ions of hydrogen or the like are implanted into a surface layer portion of a silicon wafer for SOI layer (wafer for active layer) to form an ion implanted layer and thereafter the wafer is bonded to a silicon wafer for support substrate and then the bonded wafer is exfoliated at the ion implanted layer through a heat treatment to form SOI layer (smart cut method), and so on.
However, the grinding-polishing method has a problem that the thickness uniformity of the active layer is poor (±30% or more). On the other hand, the method using oxygen ion implantation (SIMOX) has a problem that SOI structures having different crystal orientations can not be produced so as to interleave the insulating layer.
In order to solve the above problems, the inventors have already developed a process combining the oxygen ion implantation method and the grinding-polishing method as disclosed in JP-A-H05-291543.
Besides, a production method of a bonded wafer having the following construction has been proposed in JP-A-2008-016534 (published on Jan. 24, 2008) as the process combining the oxygen ion implantation method and the grinding-polishing method. That is, it is a method for producing a bonded wafer by directly bonding a wafer for active layer provided or not provided on its surface with an insulting layer to a wafer for support layer and then thinning the wafer for active layer, which comprises time-orientedly combining a step of implanting oxygen ions into a wafer for active layer to form an oxygen ion implanted layer in the active layer, a step of subjecting the wafer for active layer to a heat treatment at a temperature of not lower than 1100° C. in a non-oxidizing atmosphere, a step of bonding the wafer for active layer to a wafer for support layer, a step of conducting a heat treatment for increasing a bonded strength, a step of grinding a portion of the wafer for active layer in the resulting bonded wafer short of the oxygen ion implanted layer, a step of further polishing or etching the wafer for active layer to expose the oxygen ion implanted layer, a step of oxidizing the bonded wafer to form an oxide film on the exposed surface of the oxygen ion implanted layer, a step of removing the oxide film, and a step of heat-treating at a temperature of not higher than 1100° C. in a non-oxidizing atmosphere.
By such a combination method it is made possible to directly provide a bonded wafer being excellent in the thickness uniformity of the active layer and relatively less in the defects as evaluated by a transmission electron microscope (TEM).
However, the following problems still remain in the technique disclosed in JP-A-H05-291543:    (1) Flaws are caused on the back face of the wafer for active layer during the grinding;    (2) Flaws are caused on the front face when the oxygen ion implanted layer is exposed by grinding or when the grinding is stopped at the oxygen ion implanted layer; and    (3) When the oxygen ion implanted layer is exposed by grinding or etching, or when the grinding or etching is stopped at the oxygen ion implanted layer, a thickness of a top layer in the vicinity of an outer peripheral portion of the wafer becomes non-uniform.